1. Field of the Invention
The present invention relates generally to protective electrical wiring devices, and particularly to arc fault circuit interrupter devices.
2. Technical Background
An arc fault circuit interrupter (AFCI) is a protective device for protecting wiring from sputtering arcing conditions which, if allowed to persist could ignite nearby combustibles. Series arc faults occur in a single conductor in series with an electrical load. They may occur at a loose terminal screw or where a conductor has been severed but the ends are almost touching. The series arc current is no greater than the load current. Since the load current is limited by a 15 A or 20 A fuse or circuit breaker and since series arc faults involving loads less than 5 A are not considered hazardous, the range of load currents is 5 A to 30 A. On the other hand, parallel arc faults occur across the line, whether between the phase conductor to a neutral conductor, two phase conductors, or a phase to ground conductor. An over-driven staple or a nail driven through a cable are examples of how to get a parallel arc fault condition. Parallel arc fault current is limited by the voltage source impedance which depends on the length of conductors between the voltage source and the position of the fault in the branch circuit. Since the current is not limited by a load impedance, the range of currents is 75 A to 500 A.
AFCI's are typically located in a service panel where they are combined with a circuit breaker or in an outlet box where they are combined with a wiring device. Wiring devices comprise receptacles, ground fault circuit interrupters (GFCIs), transient voltage surge suppressors (TVSS's), surge protective devices (SPD's), protective devices, switches, occupancy sensors, nightlights, USB ports, low voltage portals, and various combinations thereof. For the wiring device-type AFCI, a portion of the branch circuit lies between the panel and the line terminals of the device referred to as the upstream circuit, and another between the load terminals of the device and the wiring connected to it and the loads that may terminate that wiring, referred to as the downstream circuit. The load terminals may include integral face receptacles and/or feed-through terminals. Wires connected to the feed-through load terminals daisy-chain to other receptacles to complete the branch circuit. Loads may be hard-wired to the AFCI or may have integral attachment plugs that receive power from the face receptacles or from downstream receptacles daisy-chained to the feed-through terminals. Loads may be connected to the receptacles using extension cords, power adapters, multiple outlet strips, or the like.
As implied by its name, an AFCI includes a circuit interrupter to interrupt the arc fault once detected. For wiring device AFCIs, it is possible to interrupt series arc faults in branch circuit wiring upstream of the AFCI and downstream of the AFCI as well as parallel arc faults downstream of the AFCI, but providing protection from parallel upstream arc faults has been a problem. Even if the AFCI could detect the upstream parallel arc fault condition and its circuit interrupter were to open as a result, the fault current would continue to flow.
In one approach that has been tried, the wiring device AFCI is required to be installed at the first outlet of the branch circuit. The downstream wiring to downstream receptacles would then be protected from parallel arc faults. Unfortunately the length of wire between the panel and the first outlet (sometimes referred to as the home run) remains unprotected from parallel arc faults. The problem is thus lessened but not eliminated.
In another approach, the length of the homerun has been limited to being a short distance, but this is burdensome to the installer and once again, the problem is not entirely eliminated.
In another approach, the conventional circuit breaker in the panel has been looked to as providing that missing protection. Although parallel arc fault currents in the homerun are greater than the 15 A or 20 A handle rating of the circuit breaker, the breaker does not always interrupt the current through the parallel arc fault quickly enough to prevent a fire.
What is needed is a wiring device AFCI that provides upstream parallel arc fault protection. What is needed is a wiring device that warns the user of the presence of an end-of-life condition when it is no longer capable of providing upstream parallel arc fault protection.